DEV: Add more structure for admin plugin config nav (#26707)

* Simplify config nav link generation to always inject the Settings
  tab
* Auto-redirect to the first non-settings config link (if there is one)
  when the user lands on /admin/plugins/:plugin_id
* Add `extras` to admin plugin serializer so plugins can add more
  data on first load
* Add PikadayCalendar page object for system specs, extracted from the
CalendarDateTimePicker to make it more generic.

spec/system/page_objects/components/calendar_date_time_picker.rb

@@ -3,25 +3,17 @@
 module PageObjects
   module Components
     class CalendarDateTimePicker < PageObjects::Components::Base
+      delegate :select_day, :select_year, to: :@pikaday_calendar
+
       def initialize(context)
         @context = context
+        @pikaday_calendar = PageObjects::Components::PikadayCalendar.new(context)
       end
 
       def component
         find(@context)
       end
 
-      def select_day(day_number)
-        component.find("button.pika-button.pika-day[data-pika-day='#{day_number}']").click
-      end
-
-      def select_year(year)
-        component
-          .find(".pika-select-year", visible: false)
-          .find("option[value='#{year}']")
-          .select_option
-      end
-
       def fill_time(time)
         component.find(".time-picker").fill_in(with: time)
       end

spec/system/page_objects/components/pikaday_calendar.rb

@@ -0,0 +1,88 @@
+# frozen_string_literal: true
+
+module PageObjects
+  module Components
+    class PikadayCalendar < PageObjects::Components::Base
+      attr_reader :context
+
+      def initialize(context)
+        @context = context
+      end
+
+      def component
+        find(@context)
+      end
+
+      def open_calendar
+        component.click
+      end
+
+      def visible_pikaday
+        find(".pika-single:not(.is-hidden)")
+      end
+
+      def hidden?
+        page.has_no_css?(".pika-single:not(.is-hidden)")
+      end
+
+      def select_date(year, month, day)
+        open_calendar
+        select_year(year)
+        select_month(month)
+        select_day(day)
+      end
+
+      def select_day(day_number)
+        find("button.pika-button.pika-day[data-pika-day='#{day_number}']:not(.is-disabled)").click
+      end
+
+      # The month is 0-based. Month name can be provided too.
+      def select_month(month)
+        parsed_month =
+          begin
+            Integer(month)
+          rescue StandardError
+            nil
+          end
+
+        if parsed_month.nil?
+          parsed_month =
+            {
+              "january" => 0,
+              "february" => 1,
+              "march" => 2,
+              "april" => 3,
+              "may" => 4,
+              "june" => 5,
+              "july" => 6,
+              "august" => 7,
+              "september" => 8,
+              "october" => 9,
+              "november" => 10,
+              "december" => 11,
+            }[
+              month.downcase
+            ]
+        end
+
+        # visible: false is here because pikaday sets the controls
+        # to opacity: 0 for some reason.
+        visible_pikaday
+          .find(".pika-select-month", visible: false)
+          .click
+          .find("option[value='#{parsed_month}']")
+          .click
+      end
+
+      def select_year(year)
+        # visible: false is here because pikaday sets the controls
+        # to opacity: 0 for some reason.
+        visible_pikaday
+          .find(".pika-select-year", visible: false)
+          .click
+          .find("option[value='#{year}']")
+          .click
+      end
+    end
+  end
+end